We have studied programmed cell death in the nervous system and the biochemical mechanism of apoptosis in general. To approach the nervous system, more sensitive and in situ methods are needed to identify cells undergoing programmed cell death. We have developed new methods to identify apoptotic cells under the microscope. This new method allowed us to identify apoptosis as the method of cerebellar granule cell death after MPP+ treatment in vitro. (1) We have also developed a molecular detection method to measure DNA strand breaks in situ. This allows us to examine brains of animals undergoing neurodegenerative changes during ischemia, MPTP treatment, and during development. This new method has allowed us to determine the role apoptosis plays during development and during various disease states of the nervous system. (2) We have developed monoclonal antibodies against apoptosis regulatory genes Bcl-2, Bcl-x, and Bax to probe the regulation of apoptosis in vitro and in vivo. The antibodies reveal that Bax migrates from the cytosol to mitochondrial membranes during apoposis. To explore Bax, Bcl-2 and Bcl-xl trafficking in neurons the green flourescent protein has been used to tag and follow the proteins during apoptosis. (3) We discovered that Bax and Bcl-xl move from the cytosol to the mitochendria as an essential step in their mechanism of apoptosis control. (4) We found RNaseL to modiate virus and interferon induced apoptosis.